Browsing by Subject "Nanocrystals"

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Aspects of bottom-up engineering : synthesis of silicon nanowires and Langmuir-Blodgett assembly of colloidal nanocrystals
(2010-08) Patel, Reken Niranjan; Korgel, Brian Allan, 1969-; Chelikowsky, James R.; Ekerdt, John G.; Mullins, C. B.; Tutuc, Emanuel
Central to the implementation of colloidal nanomaterials in commercial applications is the development of high throughput synthesis strategies for technologically relevant materials. Solution based synthesis approaches provide the controllability, high throughput, and scalability needed to meet commercial demand. A flow through supercritical fluid reactor was used to synthesize silicon nanowires in high yield with production rates of ~45 mg/hr. The high temperature and high pressure of the supercritical medium facilitated the decomposition of monophenylsilane and seeded growth of silicon nanowires by gold seeds. Crystalline nanowires with diameters of ~25 nm and lengths greater than 20 [micrometers] were routinely synthesized. Accumulation of nanowires in the reactor resulted in deposition of a conformal amorphous shell on the crystalline surface of the wire. X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and energy dispersive X-ray spectroscopy were used to determine the shell composition. The shell was identified as polyphenylsilane formed by polymerization of the silicon precursor monophenylsilane. A post synthesis etch was developed to remove the shell while still maintaining the integrity of the crystalline silicon nanowire core. Subsequent surface passivation was achieved through thermal hydrosilylation with a terminal alkene. The development colloidal nanomaterials into commercial applications also requires simple and robust bottom-up assembly strategies to facilitate device fabrication. A Langmuir-Blodgett trough was used to assemble continuous monolayers of hexagonally ordered spherical nanocrystals over areas greater than 1 cm². Patterned monolayers and multilayers of FePt nanocrystals were printed onto substrates using pre-patterned polydimethylsiloxane (PDMS) stamps and a modified Langmuir Schaefer transfer technique. Patterned features, including micrometer-size circles, lines, and squares, could be printed using this approach. The magnetic properties of the printed nanocrystal films were also measured using magnetic force microscopy (MFM). Room temperature MFM could detect a remanent (permanent) magnetization from multilayers (>3 nanocrystals thick) films of chemically-ordered L1₀ FePt nanocrystals. Grazing incidence small angle X-ray scattering was used to quantitatively characterize the grain size, crystal structure, lattice disorder, and edge-to-edge spacing of the nanocrystal films prepared on the Langmuir-Blodgett trough both on the air-water interface and after transfer.
Aspects of colloidal nanocrystals: patterning, catalysis and doping
(2005) Stowell, Cynthia Ann; Korgel, Brian Allan
Colloidal nanocrystals have many advantages over those synthesized by other means due to the flexibility not only in synthesis conditions but in post-synthesis assembly. Three aspects of colloidal nanocrystals that demonstrate this versatility were studied: the self-assembled patterning of nanocrystals into arrays through the use of fluid dynamics, the catalytic properties of nanocrystals as a function of ligand type and reaction cycle, and the doping of III-V semiconductor nanocrystals with magnetic atoms during colloidal synthesis. While much work has been done on the thermodynamically-driven formation of monodisperse or bi-modal nanocrystal superlattices, another option exists for nanocrystal self-assembly: formations driven by fluid dynamics. Hexagonal networks of gold nanocrystals were observed after drop casting gold nanocrystals in chloroform on different substrates. The honeycomb-shaped structures were calculated to be created by surface tension driven (Marangoni) convection. The honeycomb networks have a lattice parameter of 4.3 µm and their formation is highly dependant not only particle size and size distribution but concentration of particles within solution. A new synthesis for iridium nanocrystals was developed. The iridium particles could be synthesized with any of five stabilizing molecules. Taking advantage of this fact, the effect of capping ligands on the catalysis of 1-decene hydrogenation was studied. Ligands that stabilized the iridium well prevented hydrogenation while “weak” capping ligands allowed the iridium nanocrystals to reach turnover rates as high as 270 s-1. Recycling the catalytic particles also affected the activity, as the turnover frequency increased with each cycle until the particle began to agglomerate and fall out of solution. MnxIn1-xAs and MnxIn1-xP nanocrystals ranging from 2 to 10 nm in diameter were synthesized with up to xMn=0.025 for InMnAs and xMn=0.11 for InMnP. Surface exchange and magnetic measurements confirmed that much of the dopant resides in the nanocrystal core and modifies the magnetic properties of the host material through antiferromagnetic superexchange interactions. The effective Bohr magnetons of Mn in the synthesized InMnAs nanocrystals ranged from 2.2 to 3.7 µ B Mn atom, and from 3.4 to 5.1 µ B Mn atom for InMnP, values below the theoretical value of 5.9 µ B Mn atom. This result is attributed to antisite defects and interstitial doping.
CdTe/CdSe/CdTe heterostructure nanorods and I-III-VI₂ nanocrystals: synthesis and characterization
(2009-12) Koo, Bonil; Korgel, Brian Allan, 1969-
Semiconductor nanocrystals are interesting candidates as new light-absorbing materials for photovoltaic (PV) devices. They can be dispersed in solvents and cheaply deposited at low-temperature on various substrates. Also, the nanocrystals have unique optical properties depending on their size due to the quantum size effect and moreover it is easy to uniformly control their stoichiometry. CdTe/CdSe/CdTe heterostructure nanorods and I-III-VI₂ nanocrystals were selected to synthesize and investigate in order to utilize the benefits of colloidal nanocrystals described above. Colloidal nanorods with linear CdTe/CdSe/CdTe heterojunctions were synthesized by sequential reactant injection. After CdTe deposition at the ends of initially formed CdSe nanorods, continued heating in solution leads to Se-Te interdiffusion across the heterojunctions and coalescence to decreased aspect ratio. The Se-Te interdiffusion rates were measured by mapping the composition profile using nanobeam energy dispersive X-ray spectroscopy (EDS). The rate of nanorod coalescence was also measured and compared to model predictions using a continuum viscous flow model. The synthetic method of monodisperse chalcopyrite (tetragonal) CuInSe₂ nanocrystals was also developed. The nanocrystals have trigonal pyramidal shape with one polar and three non-polar surface facets. When drop-cast onto carbon substrates, the nanocrystals self-assemble into close-packed monolayers with triangular (honeycomb) lattice structure. Moreover, the effect of excess Cu precursor (CuCl) was studied for the formation of monodisperse trigonal pyramidal CuInSe₂ nanocrystals. The formation mechanism of monodisperse trigonal pyramidal CuInSe₂ nanocrystals was suggested with regard to excess amount of CuCl precursor, based on the nucleationgrowth model of colloidal nanocrystal formation. A new wurtzite phase of CuInS₂, CuInSe₂, and Cu(InxGa1-x)Se₂ (CIGS) was observed in nanocrystals synthesized by heating metal precursors and Se-(or S-)urea in alkylamine. X-ray diffraction (XRD) showed the predominant phase to be wurtzite (hexagonal) instead of chalcopyrite (tetragonal). High resolution transmission electron microscopy (TEM), however, revealed polytypism in the nanocrystals, with the wurtzite phase interfaced with significant chalcopyrite domains.
Colloidal nanocrystals with near-infrared optical properties : synthesis, characterization, and applications
(2011-12) Panthani, Matthew George; Korgel, Brian Allan, 1969-; Dodabalapur, Ananth; Chelikowsky, James; Mullins, C. Buddie; Manthiram, Arumugam
Colloidal nanocrystals with optical properties in the near-infrared (NIR) are of interest for many applications such as photovoltaic (PV) energy conversion, bioimaging, and therapeutics. For PVs and other electronic devices, challenges in using colloidal nanomaterials often deal with the surfaces. Because of the high surface-to-volume ratio of small nanocrystals, surfaces and interfaces play an enhanced role in the properties of nanocrystal films and devices. Organic ligand-capped CuInSe2 (CIS) and Cu(InXGa1-X)Se2 (CIGS) nanocrystals were synthesized and used as the absorber layer in prototype solar cells. By fabricating devices from spray-coated CuInSe nanocrystals under ambient conditions, solar-to-electric power conversion efficiencies as high as 3.1% were achieved. Many treatments of the nanocrystal films were explored. Although some treatments increased the conductivity of the nanocrystal films, the best devices were from untreated CIS films. By modifying the reaction chemistry, quantum-confined CuInSeXS2-X (CISS) nanocrystals were produced. The potential of the CISS nanocrystals for targeted bioimaging was demonstrated via oral delivery to mice and imaging of nanocrystal fluorescence. The size-dependent photoluminescence of Si nanocrystals was measured. Si nanocrystals supported on graphene were characterized by conventional transmission electron microscopy and spherical aberration (Cs)-corrected scanning transmission electron microscopy (STEM). Enhanced imaging contrast and resolution was achieved by using Cs-corrected STEM with a graphene support. In addition, clear imaging of defects and the organic-inorganic interface was enabled by utilizing this technique.
Colloidal nanocrystals: synthesis and shape-control, interparticle interactions & self-assembly
(2005) Saunders, Aaron Edward; Korgel, Brian A.
Control over nanocrystal growth kinetics provides a powerful way of tailoring particle size and shape during synthesis. Investigations into the growth of gold nanocrystals demonstrated how reaction conditions can be adjusted to control the growth rate and produce monodisperse particles. Kinetic control during the synthesis of CdS, CdSe and CdTe nanoparticles allows the shape to be tuned, from rods to spheres, without modifying the reaction chemistry. The growth and optical properties of these shapeanisotropic semiconductor particles were studied, and these methods were extended to produce semiconductor heterostructure nanorods. Solvent-mediated interparticle interactions between nanocrystals dispersed in toluene and in supercritical carbon dioxide were also studied. Nanocrystal dispersions were characterized using small-angle X-ray scattering in order to obtain information about the pair interaction potential. In organic solvents, subtle differences in the concentration-dependent scattering from dispersions allowed second virial coefficients to be measured as a function of nanocrystal size. Interestingly, larger nanocrystals exhibited overall repulsive interactions, while smaller nanocrystals exhibited attractive interactions, which is likely due to differences in ligand coverage among the different sized particles. Nanocrystals coated with fluorinated ligands could be dispersed into supercritical carbon dioxide, and the relatively strong interparticle interactions were measured at different carbon dioxide densities. As expected, the interaction strength increased as the solvent density was lowered, due to a decreased ability of the solvent to solvate the capping ligands. The formation of metastable nanocrystal flocculates was also observed at all system conditions studied. The assembly of nanocrystals into ordered superlattices under equilibrium conditions is strongly influenced by nanocrystal interparticle interactions. The formation of binary superlattices was studied, and an ordered AB phase was observed from the coassembly of small gold and large iron nanocrystals. A non-equilibrium route, breath figure templating, was also used to produce nanocrystal films with hierarchal order and porous polymer films. Evaporation of a nanocrystal or polymer dispersion in a humid atmosphere causes water droplets to nucleate and grow at the solvent-air interface. The solute stabilizes the water droplets which assemble into ordered arrays to template the drying film. The design rules for producing macroporous nanocrystal and polymer films are discussed.
Controlled synthesis and characterization of silicon nanocrystals
(2004) Pell, Lindsay Erin; Korgel, Brian Allan
In response to the demand for shrinking feature sizes and faster electronics, many resources have been dedicated to the research of nanotechnology. At present, silicon is undoubtedly the building block and key to the microelectronics world. In its bulk form, silicon is an inefficient emitter in the infrared, but as its dimensions shrink to the nanoscale, silicon exhibits unique optical and electrical properties such as size tunable photoluminescence. The more successful methods of the synthesis of silicon nanocrystals include laser ablation and silane pyrolysis; however these methods offer little in the way of particle stabilization which would prevent oxidation and allow for manipulation through dispersion in organic solvents. A novel supercritical fluid synthesis is investigated with respect to various silicon precursors such as diphenylsilane, silicon tetrachloride and trisilane. The electrochemical and luminescent properties of silicon nanocrystals, synthesized via the thermal decomposition of diphenylsilane, were studied. Differential pulse voltammetry of silicon nanocrystals in DMF and acetonitrile exhibit quantized double layer charging as previously reported for Au and CdS nanocrystals. Additionally, electron transfer reactions between positively and negatively charged nanocrystals (or between charged nanocrystals and molecular redoxactive coreactants) occurred that led to electron and hole annihilation, producing visible light. The electrogenerated chemiluminescence spectra exhibited a peak red shifted from the photoluminescence maximum. Single nanocrystal photoluminescence was investigated via Argon laser excitation and confocal microscopy. The single nanocrystals demonstrate stochastic single-step “blinking” behavior and size-dependent PL spectra with line widths approximately only three times greater than those measured for CdSe nanocrystals at room temperature. Investigation of trisilane as a viable silicon precursor in a supercritical fluid synthesis led to the formation of well formed, sub-micron, amorphous silicon colloids in high yield. Manipulation of temperature, pressure and precursor concentration allowed for the synthesis of amorphous silicon particles 60-400 nm in diameter. Polydisperse samples exhibited two dimensional, longviii range orientational order in the absence of translational order which has been compared to the Reverse Brazil Nut Effect. Additionally, metal induced crystallization was observed in amorphous silicon particles annealed in a vacuum evaporator.
CuInSe₂ nanowires and earth-abundant nanocrystals for low-cost photovoltaics
(2013-05) Steinhagen, Chet Reuben; Korgel, Brian Allan, 1969-
Widespread commercialization of photovoltaics (PVs) requires both higher power conversion efficiencies and low-cost, high throughput manufacturing. High efficiencies have been achieved in devices made from materials such as CuIn[subscript x]Ga₁₋[subscript x]Se₂ (CIGS). However, processing of these solar cells still requires high temperature and vacuum, driving up cost. A reduction in manufacturing costs can be achieved by utilizing colloidal nanocrystals. Semiconductor nanocrystals can be dispersed in solvents and deposited via simple and scalable methods under ambient conditions to form the absorber layer in low-cost solar cells. Efficiencies of ~3% have been achieved with CIGS nanocrystal PVs, but this must be improved substantially for commercialization. These devices suffer from poor charge transport in the nanocrystal layer. Here, the synthesis of nanowires and their utilization in solar cells was explored as a way to improve charge transport. CuInSe₂ (CIS) nanowires were synthesized via the solution-liquid-solid method. PV devices were fabricated using the nanowires as the light absorbing layer, and were found to exhibit a measureable power output. Earth-abundant materials were also explored, motivated by the material availability concerns associated with CIGS. Pyrite FeS₂ nanocrystals were synthesized via an arrested precipitation reaction to produce phase-pure particles 15 nm in size. These nanocrystals were spray coated to form the active layer in several different common device architectures. These devices failed to produce any power output. The material was determined to be slightly sulfur deficient, leading to a high carrier concentration and metallic behavior in the thin films, with conductivities measured to be ~5 S/nm. A nanocrystal synthesis of Cu₂ZnSnS₄ (CZTS) was also developed to produce highly dispersible crystalline particles ~11 nm in size. These nanocrystals were spray coated onto glass substrates to form the absorber layer in test PV devices, and an efficiency of 0.23% was achieved without high-temperature or chemical post-processing. Additional studies included the synthesis of CZTS nanorods and their incorporation into functioning solar cells. The selenization of CZTS nanocrystal films was also studied as a way to improve solar cell performance. High temperature annealing in a Se atmosphere was found to produce CZTS(Se) layers, which could be used in working PV devices.
Design of novel catalysts by infusion of presynthesized nanocrystals into mesoporous supports
(2008-08) Gupta, Gaurav, Ph. D.; Johnston, Keith P., 1955-
Traditionally, supported metal catalysts have been synthesized by reduction of precursors directly over the support. In these techniques, it is challenging to control the metal cluster size, composition and crystal structure. Herein, we have developed a novel approach to design catalysts with controlled morphologies by infusing presynthesized nanocrystals into the supports. High surface area mesoporous materials, including graphitic carbons, have been utilized for obtaining a high degree of metal dispersion to enhance catalyst stabilities and activities. Gold and iridium nanocrystals have been infused in mesoporous silica with loadings up to 2 wt % using supercritical CO₂ as an antisolvent in toluene to enhance the van der Waals interactions between nanocrystals and the silica. The iridium catalysts show high catalytic activity and do not require high temperature annealing for ligand removal, as ligands bind weakly to the iridium surface. To further enhance metal loadings to >10 % in the catalysts, short-ranged interactions between the metal nanocrystals and the support are further strengthened with weakly binding ligands to expose more of the metal surface to the support. For pre-synthesized FePt nanocrystals, coated with oleic acid and oleylamine ligands, high loadings >10 wt % in mesoporous silica are achieved, without using CO₂. The strong metal-support interactions favor FePt adsorption on the support and also enhance stability against sintering at high temperatures. High resistance to sintering favors formation of the FePt intermetallic crystal structure with <4 nm size upon thermal annealing at 700 °C. The fundamental understanding of the metal-support interactions gained from these studies is then utilized in the design of highly stable Pt and Pt-Cu electrocatalysts with controlled size, composition and alloy structure supported on graphitized mesoporous carbons for oxygen reduction. The resistance of the graphitic carbons to oxidation coupled with strong metal-support interactions mitigate nanoparticle isolation from the support, nanoparticle coalescence, Pt dissolution and subsequent Ostwald ripening and thus enhance catalyst stability. The control of the Pt nanocrystal morphology with high concentrations of highly active (111) surface leads to 25% higher activities than commercial Pt catalysts. Furthermore, the catalyst activities obtained for Pt-Cu catalysts are 4-fold higher than Pt catalysts due to strained Pt shell generated from electrochemical dealloying of copper from the nanoparticle surface.
Doped tungsten oxide nanocrystals for next generation electrochromic windows
(2019-05) Heo, Sungyeon; Milliron, Delia (Delia Jane); Korgel, Brian; Mullins, Charles; Bonnecaze, Roger; Crooks, Richard
Doped tungsten oxide (WO₃ [subscript -x]) nanocrystals (NCs) have lots of potential for next generation electrochromic windows compared to bulk thin films. The difference lies in intrinsic WO₃ [subscript -x] NC properties of shape and crystalline anisotropy with localized surface plasmon resonance absorption in the shorter wavelength near-infrared range, which can directly affect the major electrochromic performance of switching speed, optical modulation, and cycling stability. This work illustrates how doped WO₃ [subscript -x] NC properties are effectively utilized for enhancing the electrochromic performance. First, how shape anisotropic properties can generate highly porous film is studied using different aspect ratio of WO₂.₇₂ nanorods. By changing the nanorod interaction to electrostatic repulsion from solution ligand-stripping chemistry, highly porous mesoporous thin film from randomly packed nanorods is fabricated. Incorporating guest inorganic materials of niobium polyoxometalate clusters followed by chemical condensation, dual-band modulation of electrochromic films on flexible substrates are demonstrated, tackling cycling stability, and optical modulation issues. Second, how doped semiconductor NCs are effectively used for dynamic Bragg stacks with targeted performance of ‘on and off’ reflectance is studied. Dynamic reflectance tuning can affect the color tuning as well as efficient heat blocking. Judicious NC selection of indium tin oxide and WO₃ [subscript -x] NCs from mechanistic understanding of electrochemical modulation of optical properties, optimization of film processing, and reliable refractive index data from in situ ellipsometry enable accurate Bragg stack optimization from simulation and experimental realization. Third, using monoclinic WO₂.₇₂ nanorods as a model system having different size of three intracrystalline tunnel sites, we study spectroelectrochemical properties with different cation system (lithium, sodium, and tetrabutylammonium ion). In doing so, Al₂O₃ atomic layer deposition is employed to prevent electrolytes degradation and allows to study spectroelectrochemical properties. Na⁺ electrolytes system gives higher coloration efficiency than Li⁺ electrolytes and mainly capacitive charging behavior owing to its occupancy in the hexagonal tunnel sites. The results of these studies suggest general approach to improve electrochromic performance where shape and crystalline anisotropic properties of doped metal oxide nanocrystals can be effectively utilized for impacting spectroelectrochemical properties.
Electron transport in doped semiconductor nanocrystals
(2019-02-11) Staller, Corey Michael; Milliron, Delia (Delia Jane); Akinwande, Deji; Korgel, Brian; Mullins, Charles B
Electron transport through semiconductor nanocrystal (NC) systems is almost entirely understood by analogs to bulk science. The physics governing electron transport within NCs is entirely analogous to bulk semiconductors with extreme spatial constraints. In contrast, the physics of electrons conducting between NCs is understood through the physics of amorphous materials, granular metals, or bulk semiconductors, depending on the structure of the NC ensemble. Herein is an investigation of how dopant distribution engineering can be utilized to modulate near surface depletion in NC films. The dependence of NC film conductivity on dopant distribution is eliminated by surface passivation. A code to fit the optical absorption of colloidal NCs is developed to account for surface scattering, depletion, size heterogeneity, and dopant heterogeneity. This code is used to define the conduction within an individual NC. The intra-NC conduction is used as a metric to describe and define the phase diagram of NC film electron transport. Using the criteria developed here, we make metallic films in a controlled manner. This work illustrates an overview of bulk electron transport and an introduction of NC film electron transport in Chapter 1. These descriptions will then be used to investigate the powerful capability to engineer intra-NC dopant distribution to manipulate NC film conductivity in Chapter 2. The intra-NC conductance is then investigated using a novel code to fit the optical absorption of NCs in Chapter 3. With a deep understanding of intra-NC transport, the electron transport phase diagram is constructed in Chapter 4.
Electron transport, self-assembly, and electroluminescence of nanocrystal superlattices
(2003-05) Doty, Richard Christopher; Korgel, Brian Allan, 1969-
In order to assess the potential applications of nanotechnology, the fundamental properties of nanocrystals and the self-assembled arrays they form must be studied in detail. The electrical conductivity of monodisperse and polydisperse Ag nanocrystal superlattices was measured as a function of temperature. A fundamental difference between polydisperse and monodisperse nanocrystal superlattices was found. Polydisperse superlattices displayed insulating behavior throughout the entire temperature range. Monodisperse superlattices displayed a metal-insulator transition that shifted to lower temperatures for larger Ag nanocrystals. At temperatures above the metalinsulator transition, the monodisperse superlattices exhibited a positive temperature coefficient of resistance, characteristic of a metal. Below the metalvii insulator transition, the temperature coefficient of resistance was negative, characteristic of an insulator. The ability to control the formation of complex, self-assembled nanocrystal superlattices is very important for potential electrical and optical applications. With the correct concentration and size ratio, nanocrystals with a bimodal size distribution can self-assemble into LmSn structures. The formation of 2D monolayers of these complex structures was studied by performing random sequential adsorption (RSA) simulations of tethered hard disks that are able to undergo limited Monte Carlo surface diffusion. Nanocrystal size ratios of 0.155, 0.414, and 0.533 were examined. Melting simulations of perfect LmSn structures reveal that RSA kinetics frustrate superlattice ordering, creating a kinetic bottleneck to ordered LmSn structures. One of the more promising applications for nanocrystals is light emitting diodes (LEDs). Si nanocrystals synthesized by thermal decomposition of phenylsilane precursors in a supercritical hexane solution with and without the addition of octanol, which serves as a capping ligand, were used as the emitting layer in an LED. Electroluminescence from these Si nanocrystal LEDs was reddish-orange or white depending on the reaction conditions of the nanocrystal synthesis. The current-voltage behavior was characteristic of space-charge limited current, and the devices exhibited relatively low turn-on voltages (∼ 6-7 V). External quantum efficiencies varied between 10-5 and 10-4 %.
First principles calculations of Raman spectra for nanostructures and improved high order forces
(2015-12) Bobbitt, Nathaniel Scott; Chelikowsky, James R.; Demkov, Alexander A; Ekerdt, John G; Hwang, Gyeong S; Korgel, Brian A
Advances in computing technology coupled with theoretical developments on the electronic structure problem have laid the foundation for the rapidly growing field of computational materials science. Modern supercomputers are able to perform ab initio calculations of realistic systems containing thousands of atoms. This is an important step forward in the maturation of the field because computational insight can be used to make predictions about or predict experimental data. This dissertation aims to address contemporary theory and practice of solving the electronic structure problem for a variety of nanoscale systems, most of which are of interest for energy application such as photovoltaics or Li-ion batteries. Our calculations are performed within density functional theory using real-space pseudopotentials. In the first part, we examine nanocrystals. We calculate size-dependent properties for ZnO nanocrystals with Al and Ga dopants. Next, we calculate Raman spectra for Si nanocrystals with Li impurities and Si-Ge core-shell structures, which gives us insight into the structure of these nanocrystals. In the second portion, we examine in depth the calculation of interatomic forces within density functional theory and propose a new integration scheme which we demonstrate calculates more accurate bond lengths and vibrational frequencies and improves the stability of molecular dynamics simulations.
Formation of noble metal nanocrystals in the presence of biomolecules
(2007-05) Burt, Justin Lockheart, 1979-; Yacamán, M. Jose
One of the most promising, yet least studied routes for producing biocompatible nanostructures involves synthesis in the presence of biomolecules. I hypothesized that globular proteins could provide a suitable framework to regulate the formation of noble metal nanocrystals. As proof of concept, I designed two novel synthesis protocols utilizing bovine serum albumin (BSA) protein to regulate the formation of gold nanocrystals. In the first case, the standard protocol for polyol reduction was modified by replacing ethylene glycol with glycerin, replacing synthetic polymers with BSA as protecting agent, and decreasing the reaction temperature. In the second case, the BrustSchiffrin two-phase reduction was modified by replacing alkylthiols with BSA as protecting agent, which facilitated a strictly aqueous phase synthesis. Due to superior product yield and rapid reduction at room temperature, the aqueous protocol became the foundation for subsequent studies. I extended this approach to produce well-dispersed ~2nm silver, gold, and platinum nanocrystals. Having demonstrated the feasibility of BSA-functionalized nanocrystals, some potential uses were explored. BSA-functionalized silver nanocrystals were employed in a broader study on the interaction of silver nanocrystals with HIV. BSA-functionalized gold nanocrystals were utilized for in vivo dosage of a contrast enhancing agent to bacteria. BSAfunctionalized platinum nanocrystals were studied as hydrogenation catalysts. Since many intriguing uses for protein-functionalized nanocrystals involve incorporation into biosystems, I sought to enhance biocompatibility by using ascorbic acid as reducing agent. Initial experiments revealed elongated and branched nanocrystals. Such structures were not observed in previous synthesis protocols with BSA, so I hypothesized ascorbic acid was driving their formation. To test my assertion, I reduced ionic gold in an aqueous solution of ascorbic acid, thereby discovering a new method for producing multiply-branched gold nanocrystals. Two conditions were necessary to achieve multiply-branched structures: rapid kinetics, and strongly acidic pH. By exploiting ascorbic acid complexation with BSA to moderate reaction kinetics, and using sodium hydroxide to provide basic pH, the two conditions for branching were negated, and well-dispersed ~2.5nm gold nanocrystals were obtained. This protocol represents a novel, environmentally benign approach to producing biocompatible nanocrystals, relying on proteins, ascorbic acid, sodium hydroxide, and water, all at ambient temperature.
Infrared plasmonic doped metal oxide nanocubes
(2020-04-29) Cho, Shin Hum; Milliron, Delia (Delia Jane); Korgel, Brian A.; Truskett, Thomas M.; Li, Xiaoqin
Localized surface plasmon resonance (LSPR) in semiconductor nanocrystals (NCs) that results in resonant absorption, scattering, and near field enhancement around the NC can be tuned across a wide optical spectral range from visible to far-infrared by synthetically varying doping level. Cube-shaped NCs of conventional metals like gold and silver generally exhibit LSPR in the visible region with spectral modes determined by their faceted shapes. However, faceted NCs exhibiting LSPR response in the infrared (IR) region are relatively rare. We describe the colloidal synthesis of nanoscale fluorine-doped indium oxide (F:In₂O₃) cubes with LSPR response in the IR region, wherein fluorine was found to both direct the cubic morphology and act as an aliovalent dopant. The presence of fluorine was found to impart higher stabilization to the (100) facets, suggesting that the cubic morphology results from surface binding of F-atoms. In addition, fluorine acts as an anionic aliovalent dopant in the cubic bixbyite lattice of In₂O₃, introducing a high concentration of free electrons leading to LSPR. The cubes exhibit narrow, shape-dependent multimodal LSPR extinction peaks due to corner- and edge-centered modes. The spatial origin of these different contributions to the spectral response are directly visualized by electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). A synthetic challenge in faceted metal oxide NCs is realizing tunable LSPR near-field response in the IR. We expand to colloidal synthesis of fluorine, tin co-doped indium oxide (F,Sn:In₂O₃) NC cubes with tunable IR range LSPR. Free carrier concentration is tuned through controlled Sn dopant incorporation, where Sn is an aliovalent n-type dopant in the In₂O₃ lattice. Monolayer NC arrays are fabricated through liquid-air interface assembly, NC film nanocavities with heightened near-field enhancement (NFE). The tunable F,Sn:In₂O₃ NC near-field is coupled with PbS quantum dots, via the Purcell effect. The detuning frequency between the nanocavity and exciton is varied, resulting in IR near-field dependent enhanced exciton lifetime decay
Linear and nonlinear optical spectroscopies of SiGe interfaces and Si nanocrystals
(2002) Jiang, Yingying; Downer, Michael Coffin
Linear and nonlinear optical spectroscopies are used to study SiGe alloy films and Si nanocrystals (NCs). With spectroscopic ellipsometry (SE), a bulk-sensitive linear optical probe, we demonstrate in-situ monitoring and control of compositionally graded SiGe films grown on Si(001) by chemical vapor deposition. Feedback control is achieved by comparing the Ge composition of the most recently deposited layer determined from SE to the set values, then adjusting the flow of disilane gas accordingly. Second harmonic generation (SHG), a surface/interfacesensitive nonlinear optical probe, complements SE greatly in monitoring film growth. We develop a real-time SHG technique by tracking surface Ge composition with the peak of the SHG spectrum (E1 resonance) using a 15 femtosecond broad bandwidth laser. Data acquisition is much faster than traditional SHG spectroscopy, in which a 100 femtosecond narrow bandwidth laser must be tuned. Using broadband SHG and SE, we also explore the strain effect caused by adding a small amount of C into SiGe alloys. SHG studies are extended from the planar surface/interface such as SiGe/Si to the sharply curved Si/SiO2 interfaces of Si NCs embedded in SiO2. We observe SHG from 3-dimensional distributions of spherical Si NCs prepared by ionimplantation into glass, which have applications in photonic and light-emitting devices. The results suggest that SHG originates microscopically from Si/SiO2 interfaces states, which are passivated by hydrogen annealing of NC samples, and macroscopically in part from fluctuations in NC size, shape and density. We also study SHG from dense (1010 or 6×1011 cm−2 ) 2-dimensional layers of Si NC (5 or 8 nm average diameter) prepared by chemical vapor deposition of Si precursor gases onto an oxidized Si wafer, and subsequently embedded in SiO2. Such Si NC layers act as a controllable planar charge storage layer in flash-memory devices. Time-dependent SHG measures the electrostatic charging and discharging of the NC layer in real-time. By polarization-dependent and frequency-domain interferometric SHG (FDISH) spectroscopy, SHG intensity and phase spectra of Si NCs are distinguished from contributions of the Si substrate, and reveal a NC-size-dependent blue-shift of the E1 resonance, consistent with quantum confinement, that can be used as an in-situ size diagnostic. Although these results were obtained ex-situ, they show that SHG can probe key material and electrical properties of Si NCs sensitively without contacting the sample, and thus can be transferred readily to in-situ, real-time monitoring of the deposition of Si NCs.
Magnetic studies of colossal magnetoresistance materials and FePt nanocrystals
(2007-12) Hyun, Changbae, 1974-; Lozanne, Alejandro L. de
This dissertation introduces scanning probe microscopy (SPM) and describes the construction and design of a home built low temperature magnetic force microscope (MFM). Then the magnetic coatings on atomic force microscope cantilevers with a focused ion beam (FIB) will be explained. This technique allows the convenient deposition of complex or expensive materials such as CoCrPt. With the MFM tip coated by FIB, the ferromagnetic domain structure of a La[subscript 0.67]Ca[subscript 0.33]MnO₃ film is studied as a function of an in-plane magnetic field below room temperature. Next I will discuss the use of chemically-synthesized FePt nanocrystals as a good candidate for high density storage media. This nanocrystal film showed sintering problems during the annealing process, which is essential to make FePt a hard ferromagnet. A silica overcoating method was used to prevent nanocrystal sintering, which allowed the MFM study of films made from these nanocrystals. I will also discuss resistance measurements of the FePt nanocrystals.
Metal halide perovskite nanocrystals : synthesis, stability, and lead-free alternatives
(2021-05) Zhang, Yangning; Korgel, Brian Allan, 1969-; Milliron, Delia J; Humphrey, Simon M; Truskett, Thomas M
Metal halide perovskites are an emerging family of semiconductor materials with excellent optoelectronic properties suitable for applications such as solar cells and light-emitting diodes. The nanocrystals of perovskites are especially versatile due to their tunable size, shape, compositions, and size-dependent band gaps. Perovskite nanocrystals are also ideal candidates as nanoscale building blocks for the formation of superlattice assemblies, potentially giving rise to new properties depending both on the intrinsic characteristics of individual nanocrystals and on their organization in the assembly. Despite the amazing opportunities it offers, perovskite research is always faced with two major challenges, limited stability and lead toxicity. In this dissertation, I presented my efforts in addressing these two challenges in perovskites nanocrystals and their assemblies. Because the instability of nanocrystals is closely related to the synthetic process, we first provided a practical guide to the synthesis and purification of perovskite nanocrystals, emphasizing the tips and tricks in obtaining samples with better stability and reproducibility. Then, we probed the structural changes in two prototypical iodine-based perovskite systems, MAPbI₃ and CsPbI₃ nanocrystal superlattices, by in-situ X-ray scattering. The thermal-induced transformations in MAPbI₃ nanocrystals started with a tetragonal-to-cubic perovskite phase transformation at ~60 °C, followed by a chemical decomposition into PbI₂ at ~90 °C. The nanocrystal superlattice disintegrated simultaneously with the degradation of perovskite lattice. When annealed with hexane solvent vapor, CsPbI₃ nanocrystals didn’t change atomic structure, but their superlattice changed from a tetragonal lattice to a semi-disordered structure. The annealing process can be reversed by evaporating the solvent. Considering the limited stability of MAPbI₃ and CsPbI₃ nanocrystals, I designed and obtained mixed A-site Cs₁₋ₓMAₓPbI₃ nanocrystals by post-synthetic cation exchange. Cs₀.₅₅MA₀.₄₅PbI₃ nanocrystals showed enhanced thermal stability than parent MAPbI₃ and CsPbI₃ nanocrystals, suggesting that compositional alloying can be an effective strategy for obtaining more stable perovskite nanocrystals. Finally, due to the concerns with lead toxicity, Cs₂AgBiBr₆ double perovskite nanocrystals are developed as promising lead-free alternatives. We explored the roles of ligands in the synthesis of Cs₂AgBiBr₆ nanocrystals, and found that acid-to-amine ratio affects nanocrystal morphology and reaction yield. Only oleylamine remains bonded to nanocrystals after purification, and oleic acid can be replaced completely by an alkyl phosphinic acid in the reaction. Cs₂AgBiBr₆ nanocrystals were more thermally stable than lead-based perovskites, but their colloidal stability needs to be improved.
Micro solar cells, Raman spectroscopy, and flow synthesis of copper indium selenide nanocrystals
(2016-12-16) Pernik, Douglas Ryan; Korgel, Brian Allan, 1969-; Milliron, Delia J; Vanden Bout, David A; Chelikowsky, James R
Copper indium selenide nanocrystals are an attractive material for solar cell applications due to its favorable bandgap, moderate temperature synthesis, and solution processability in air. Solution processing in particular allows a whole range of new photovoltaic applications to be explored, as most current commercial solar cell technologies (bulk Si, CdTe, and CuIn [subscript 1-x] Ga [subscript x] Se₂) employ high temperatures and/or high vacuums to grow quality crystals that make up the photovoltaic absorber layer. Copper indium selenide (CuInSe₂) nanocrystals are unique in this regard in that they can be spray-deposited at room temperature in ambient conditions to form a solar cell’s light-absorbing layer. One particular application is investigated here: micro groove solar cells using flexible substrates. These solar cells are fabricated by filling micron-scale groove features with CuInSe₂ nanocrystals. The CuInSe₂ nanocrystals make contact with Au and CdS at opposite groove walls to create lateral junctions that allow charge extraction. Micro solar cells were optimized by varying the thicknesses of coatings, wall coating material thicknesses, groove angles, and ligand exchange procedures. Best-performing devices operate in the range of 3% power conversion efficiency utilizing this material set. Raman spectroscopy and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) were utilized to investigate the cationic ordering of CuInSe₂ nanocrystals. In CuInSe₂ nanocrystals, the Raman A₁ mode appears as a broad peak centered at 182 cm⁻¹, which is indicative of the sphalerite, cation-disordered structure. HAADF-STEM, on the other hand, revealed a partially ordered cationic structure in one particle, which is not typical for CuInSe₂. Synthesizing the nanocrystals at higher temperatures and annealing at high temperatures both promoted cation ordering to the chalcopyrite structure. Finally, a flow-through reactor was designed to allow the controlled scale-up synthesis of CuInSe₂ nanocrystals. This reactor utilized coiled glass tubing inside of a bulk heating element and was used to synthesize phase-pure nanocrystals.
Modeling of gold nanocrystal assemblies in superlattices and vesicles, and the synthesis of nanocrystals for low-temperature solar cell fabrication
(2015-05) Bosoy, Christian Alan; Korgel, Brian Allan, 1969-; Mullins, Charles B; Truskett, Thomas M; Milliron, Delia; Vanden Bout, David A
Recently, nanocrystal (NC) research has grown substantially, due to their unique and diverse properties. Their flexibility has led to a wide set of proposed applications, such as contrast agents in biomedical fields, ordered nanostructures for microelectronics/plasmonics, and as a cheaper alternative to chemical vapor deposition (CVD) methods. While great advancements have been made in utilizing NCs, three challenges often arise – difficulty in characterizing complex nano-systems, a lack of theoretical exploration as to how or why nanocrystals assemble, and challenges in exploiting the benefits of nanocrystals while minimizing disadvantageous properties. This dissertation will address each of these issues in specific systems. First, thorough work has been done suggesting that hydrophobic gold nanocrystals can be encapsulated in vesicle bilayers. However, the primary characterization method for this system is Cryo-Transmission Electron Microscopy, which cannot provide adequate resolution and contrast to fully characterize nanostructures. Here, small angle x-ray scattering is explored as a method for revealing detailed information regarding the bilayer structure. vii Next gold nanocrystal superlattices are explored through molecular dynamics (MD) simulations. While many works have shown crystal structure transitions in a variety of systems, a detailed explanation as to why certain crystal structures are preferred has yet to be provided. This work offers detailed MD simulations to reveal details regarding the packing density of various crystal structures and to estimate diffusion coefficients in various packings. Furthermore, the free energy difference between BCC and FCC configurations for a small set of gold nanocrystals is explored by thermodynamic integration. The simulated properties are also compared to a small set of real systems. The second half of this dissertation addresses practical applications of NCs for photovoltaics. Despite manufacturing benefits, it is well known that the small NC grains and insulating capping ligands make it difficult to produce efficient solar cells. Therefore, two approaches to removing these ligands and growing nanocrystal grains are explored. The first approach focuses on further studying CuInSe2 synthesis as a window into grain growth. The second offers an example of a material with favorable properties for grain growth – Cu3BiS3 – and addresses difficulties in producing it.
Non-volatile memory devices beyond process-scaled planar Flash technology
(2007-12) Sarkar, Joy, 1977-; Banerjee, Sanjay; Gleixner, Robert J.
Mainstream non-volatile memory technology dominated by the planar Flash transistor with continuous floating-gate has been historically improved in density and performance primarily by means of process scaling, but is currently faced with significant hindrances to its future scaling due to fundamental constraints of electrostatics and reliability. This dissertation is based on exploring two pathways for circumventing scaling limitations of the state-of-the-art Flash memory technology. The first part of the dissertation is based on demonstrating a vertical Flash memory transistor with nanocrystal floating-gate, while the second part is based on developing fundamental understanding of the operation of Phase Change Memory. A vertical Flash transistor can allow the theoretical minimum cell area and a nanocrystal floating-gate on the sidewalls is shown to allow a thinner gate-stack further conducive to scaling while still providing good reliability. Subsequently, the application of a technique of protein-mediated assembly of preformed nanocrystals to the sidewalls of the vertical Flash transistor is also demonstrated and characterized. This technique of ordering pre-formed nanocrystals is beneficial towards achieving reproducible nanocrystal size uniformity and ordering especially in a highly scaled vertical Flash cell, rendering it more amenable to scaling and manufacturability. In both forms, the vertical Flash memory cell is shown to have good electrical characteristics and reliability for the viability of this cell design and implementation. In the remaining part of this dissertation, studies are undertaken towards developing fundamental understanding of the operational characteristics of Phase Change Memory (PCM) technology that is expected to replace floating-gate Flash technology based on its potential for scaling. First, a phenomenon of improving figures of merit of the PCM cell with operational cycles is electrically characterized. Based on the electrical characterization and published material characterization data, a physical model of an evolving "active region" of the cell is proposed to explain the improvement of the cell parameters with operational cycles. Then, basic understanding is developed on early and erratic retention failure in a statistically significant number of cells in a large array and, electrical characterization and physical modeling is used to explain the mechanism behind the early retention failure.